This might get a bit long, but:
Clarify your message. List out your top 3 professional accomplishments since your last raise (or in a hiring process, the 3 reasons why they want you). Know that it’s okay to repeat them over the course of negotiations. Stay on message. This will also help you stay psyched up.
Conversely, I found it really helpful to brainstorm all the objections I could think of, from the employer’s perspective, and think of how I could respond. Soon it became clear that there were 3–4 main arguments that I could use to address a bunch of different hesitations my boss might have. From there I practiced a couple of very simple scripts that I was comfortable with, to help my conflict-averse ass be more assertive. I love scripts, 10/10.
I find costuming very important. POWER-DRESSING FOR SUCCESS. It’s very stupid but I find it affects my confidence a lot.
I also brainstormed a few other things I could ask for if there was less money available than I wanted. Work from home arrangements? Flex hours? More vacation time? Professional development opportunities? An ergonomic keyboard?
Finally, in prep for conversations like these I like to set myself one ultimate goal for the conversation: when I started it was as simple as “don’t agree to the first offer,” because that alone was such a challenge for me that whatever else happened, if I did that I would have made progress. These days it’s a bit more ambitious, but I still find it really helpful to have one single non-negotiable goal for myself. It’s a good personal barometer for success, even if you don’t get everything you want in negotiations.
Remember: you can always end the conversation with, “Okay, let me go do some math / think about it / crunch some numbers and I’ll get back to you tomorrow.” Buy yourself time!!